Phosphate coating composition



United States Patent 3,269,877 Ice Patented August 30, 1966 This invention relates to phosphate coating compositions, and more particularly it relates to a novel phosphate coating composition for ferrous metal surfaces which permits reduced coating times, improved coating appearance and improved corrosion resistance.

Among the criteria determining the acceptability of phosphate coatings on ferrous metal surfaces are (1) the time required to deposit the coating, (2) the corrosion resistance of the deposited coating and (3) the appearance of the finished coating. It is known that the more rapidly that a phosphate coating having acceptable corrosion resistance can be deposited on a ferrous metal surface, the more economically attractive the coating composition will be. Furthermore, in the phosphate coating trade, a coating color of from dark gray to black is highly desirable. It is therefore an object of this invention to provide a phosphate coating composition for ferrous metal surfaces. which will produce a dark gray to black coating, which composition will effectively produce this coating in a short period of time, with excellent corrosion resistance attributable to the coating.

It is known that a mild steel panel may be provided with a corrosion resistant phosphate coating thereon by the immersion thereof in a solution containing a combination of phosphate ion, nitrate ion and zinc ion. An example of the use of such a coating composition is shown in the following example.

Example 1 An aqueous solution having the following composition was prepared:

Weight percentage of ion in total solution Phosphate ion (H PO 0.9 Nitrate ion (HNO 0.284 Zinc ion (ZnO) 0.351

A clean, standard mild steel panel was immersed in this solution for a period of 15 minutes, the solution having a temperature in the range of from 190 to 200 F. The panel was then rinsed and treated with a final chromic acid rinse, said rinse having a level of 0.03% hexavalent chrome, a FAP of 0.3 and a TAP of 1.3. The chrome rinse was maintained at a temperature of 150 F. The rinsed and dried panel had a light gray appearance, and when subjected to an ASTM B11761 salt spray corrosion test, a nine hour salt spray protection for said coating was indicated.

Another example of the use of the formerly known phosphate coating composition is as follows:

Example 2 The procedure of Example 1 was repeated, except that instead of the chromium rinse a rust preventive emulsion (Detrex 521) was applied to the coated panel at a 30% dilution in water at 150 F. The resultant panel was likewise subjected to the ASTM B117-61 salt spray corrosion test and indicated 96 hours of protection.

It has now been discovered that by incorporating a compound producing a cobalt ion into the coating compositions of the type utilized in Examples 1 and 2, a coating composition having a more rapid deposition rate, i.e. approximately 5 minutes as against a norm of about minutes is obtained. Furthermore, the coating produced by the cobalt ion containing composition is more resistant to corrosion, and such coating has a desirable dark gray to black color. The cobalt ion may be provided by any soluble salt of the metal such as cobalt chloride, cobalt nitrate, cobalt phosphate, cobalt carbonate and cobalt sulphate, for example. The anion of the cobalt compound does not appear to be involved except from the standpoint of solubility in the coating solution. The following examples show the use of such an improved coating composition containing a cobalt ion.

Example 3 An aqueous working solution having the following composition was prepared:

Weight percentage of ion in total solution Phosphate ion (H PO 0.9 Nitrate ion (HNO 0.284 Zinc ion (ZnO) 0.351 Cobalt ion (Co(NO 0.012

A clean, standard mild steel panel was immersed in this solution for a period of 15 minutes, the solution having a temperature in the range of from 190 to 200 F. The panel was then rinsed and treated with a final chromic acid rinse, said rinse having a level of 0.03% hexavalent chrome, a FAP of 0.3 and a TAP of 1.3. The chrome rinse was maintained at a temperature of 150 F. The rinsed and dried panel had a dark gray to black appearance, and when subjected to an ASTM B117-61 salt spray corrosion test, a 120 hour salt spray protection for said coating was indicated.

Example 4 An aqueous working solution having the following composition was prepared:

Weight percentage of ion in total solution Phosphate ion (B 1 0 0.9 Nitrate ion (HNO 0.284 Zinc ion (ZnO) 0.351 Cobalt ion (C0003) 0.012

A clean, standard mild steel panel was immersed in this solution for a period of 15 minutes, the solution having a temperature in the range of from 190 to 200 F. The panel was then rinsed and treated with a final chromic acid rinse, said rinse having a level of 0.03% hexavalent chrome, a FAP of 0.3 and a TAP of 1.3. The chrome rinse was maintained at a temperature of F. The rinsed and dried panel had a dark gray to black appearance, and when subjected to an ASTM B117-61 salt spray corrosion test, a 115 hour salt spray protection for said coating was indicated.

Example 5 An aqueous working solution having the following composition was prepared:

Weight percentage of ion in total solution Phosphate ion (H PO 0.9 Nitrate ion (HNO 0.284 Zinc ion (ZnO) 0.351 Cobalt ion (CoCl 0.012

A clean, standard mild steel panel was immersed in this solution for a period of 15 minutes, the solution having a temperature in the range of from to 200 F. The panel was then rinsed and treated with a final chromic acid rinse, said rinse having a level of 0.03% hexavalent chrome, a FAP of 0.3 and a TAP of 1.3. The chrome rinse was maintained at a temperature of 150 F. The rinsed and dried panel had a dark gray to black appearance, and when subjected to anASTM Bl17-61 salt spray corrosion test, a 90 hour salt spray protection for said coating was indicated.

Example 6 An aqueous working solution having the following composition was prepared:

Weight percentage of ion in total solution Phosphate ion (H P 0.9 Nitrate ion (HNO 0.284 Zinc ion (ZnO) 0.351 Cobalt ion (C080 0.012

A clean, standard mild steel panel was immersed in this solution for a period of 15 minutes, the solution having a temperature in the range of from 190 to 200 F. The panel was then rinsed and treated with a final chromic acid rinse, said rinse having a level of 0.03% hexavalent chrome, a FAP of 0.3 and a TAP of 1.3. The chrome rinse Was maintained at a temperature of 150 F. The rinsed and dried panel had a dark gray to black appearance, and when subjected to an ASTM -Bll76l salt spray corrosion test, a 96 hour salt spray protection for said corrosion test, a 100 hour salt spray protection for said coating was indicated.

Example 7 An aqueous working solution having the following composition was prepared:

Weight percentage of ion in total solution Phosphate ion (H PO 0.9 Nitrate ion (HNO 0.284 Zincion(ZnO) 0.351 Cobalt ion (Co (PO 0.012

A clean, standard mild steel panel was immersed in this solution for a period of 15 minutes, the solution having a temperature in the range of from 190 to 200 F. The panel was then rinsed and treated with a final chromic acid rinse, said rinse having a level of 0.03% hexavalent chrome, a FAP of 0.3 and a TAP of 1.3. The chrome rinse was maintained at a temperature of 150 F. The rinsed and dried panel had a dark gray to black appearance, and when subjected to an ASTM 'Bl176l salt spray corrosion test, a 100 hour salt spray protection for said coating was indicated.

Example 8 The process of Example 3 was repeated except that instead of the chromium rinse a rust preventive emulsion (Detrex 521) was applied to the coated panel at a 30% dilution in water at 150 F. The resultant panel was likewise subjected to the ASTM B 1l761 salt spray corrosion test and indicated more than 200 hours of protection.

Example 9 The process of Example 4 was repeated except that instead of chromium rinse a rust preventive emulsion (Detrex 521) was applied to the coated panel at a 30% dilution in water at 150 F. The resultant panel was likewise subjected to the ASTM B117-61 salt spray corrosion test and indicated more than 200 hours of protection.

Example 10 The process of Example was repeated except that instead of the chromium rinse a rust preventive emulsion (Detrex 521) was applied to the coated panel at a 30% dilution in water at 150 F. The resultant panel was likewise subjected to the ASTM B117-61 salt spray corrosion test and indicated 200 hours of protection.

Example 11 The procedure of Example 6 was repeated, except that instead of a chromium rinse a rust preventive emulsion (Detrex 521) was applied to the coated panel at a 30% dilution in water at 150 F. The resultant panel was likewise subjected to the ASTM B117-61 salt spray corrosion test and indicated more than 200 hours of protection.

Example 12 The procedure of Example 7 was repeated except that instead of a chromium rinse a rust preventive emulsion (Detrex 521) was applied to the coated panel at a 30% dilution in water at 150 F. The resultant panel was likewise subjected to the ASTM B 1176'l salt spray corrosion test and indicated more than 200 hours of protection.

The effective range of cobalt ion concentration in the compositions in accordance with this invention in a final use solution is from about 0.0003 to about 0.09% by weight of the total composition. A lower cobalt ion concentration is ineffective while a concentration above about 0.09% by weight results in the production of a less uniform, undesirable coating showing a tendency to form blush rust.

The dark gray to black color of the coating produced by the utilization of the compositions of this invention can be further intensified when a minimum of about 0.06% 'by weight of ferrous ion is present in the final phosphating solution. This minimum level of iron ion concentration is produced in situ rapidly when ferrous work is processed through the solution. If desired, however, ferrous ion may be added to the solution, by dissolving the appropriate amount of ferrous sulfate or other soluble ferrous salt in the solution. The level of ferrous ion is not critical until its concentration reaches the range of from about 0.5 to about 0.8% by weight of the final solution, at which point the coating quality and uniformity are negatively effective. In order to obviate such negative effect, the iron content should be reduced by the addition of chlorate, nitrite, peroxide or other oxidants, or by the reduction of the iron content by water dilution. A preferred range of cobalt ion concentration in the compositions in accordance with this invention is from about 0.001 to about 0.07% by weight of the final use solution, as seen by the results shown in the following examples.

Example 13 The procedure of Example 3 was repeated except that the cobalt ion concentration was maintained at about 0.0003% by Weight. The resultant panel was likewise subjected to the ASTM B117-61 salt spray corrosion test and indicated 12 hours of protection.

Example 14 The procedure of Example 3 was repeated except that the cobalt ion concentration was maintained at about 0.0010% by weight of the total solution, and when subjected to the ASTM Bl 17-61 salt spray corrosion test, the resultant panel was shown to have hours of protection.

Example 15 The procedure of Example 3 was repeated except that the cobalt ion concentration was maintained at about 0.0300% by weight of the total solution. The resultant panel was likewise subjected to the ASTM Bl17-61 salt spray corrosion test and indicated hours of protection.

Example 16 The procedure of Example 3 was repeated except that the cobalt ion concentration was maintained at about 0.0700% by weight of the total solution. The resultant panel was likewise subjected to the ASTM B117-61 salt spray corrosion test and indicated hours of protection.

Example 17 The procedure of Example 3 was repeated except that the cobalt ion concentration was maintained at about 0.0900% by weight of the total solution. The resultant panel was likewise subjected to the ASTM B1l7-61 salt spray corrosion test and indicated 15 hours of protection.

Example 18 A clean, standard mild steel panel was immersed in the working solution described in Example 3, and it was determined that after only 5 minutes at 190 F. a high quality uniform dark gray to black colored coating was produced on the panel. Under identical conditions, but with the cobalt ion absent from such working bath, very poor coatings were produced in 5 minutes, and these coatings showed an incipient form of corrosion, referred to in the industry as blush rust. In the absence of the cobalt .ion, exposure times in excess of at least minutes are required to produce an acceptable coating free of such blush rust.

It will be apparent from the showing of Example 18 that a substantial reduction in coating time is possible by the use of a novel coating composition in accordance with this invention, which reduction in time is of substantial benefit in permitting a shorter overall processing time industrially.

Having thus described our invention, we claim:

An aqueous phosphate coating composition consisting essentially of a water solution containing a phosphate ionproducing agent, a nitrate ion-producing agent, a zinc ionproducing agent, a cobalt ion-producing agent, and a ferrous ion-producing agent, wherein the amount of effective 6 cobalt ion is in the range of about 0.0003% by weight to about 0.09% by weight of the total solution, and said ferrous ion concentration is within the range of from about 0.06% by weight to about 0.5% by weight of the total solution.

References Cited by the Examiner UNITED STATES PATENTS 2,121,574 6/1938 Romig 148-615 2,296,844 9/1942 Glasson 1486.15 2,346,302 4/1944 Hays et a1. 2,487,137 11/1949 Hoover et al 148-6.15 2,540,314 2/1951 Amundsen 148-615 3,240,633 3/1966 Gowman 148--6.15

FOREIGN PATENTS 731,882 6/1955 Great Britain. 757,050 9/ 1956 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

R. S. KENDALL, Assistant Examiner. 

